232 MAYOR— THE REEFS OF TUTUILA, SAMOA. 



before Pliocene times (Vaughan, T. W., 1917, U. S. Geological Sur- 

 vey, Professional Papers, 98 T, pp. 355-376). According to Daly, 

 coral growth was retarded during periods of lowered sea level and 

 coincide with periods of glaciation when water was taken out of the 

 oceans to constitute the continental ice sheets ; but in Tutuila we 

 have evidence that at the time of highest ocean level coral reefs did 

 not grow around the island. 



However, Ulrich (1920, Journal IVashington Acad. Sci., Vol. 

 10, pp. 57-78) states that the pressure of an ice sheet may depress 

 the level of the interior of a continent, and this movement tends to 

 be compensated by an elevation in the strand-line. Thus the rise 

 in the continental shelf and of the coastal waters may more than 

 compensate for the water taken out of the ocean to form the ice 

 sheet, and there may even be a rise of sea level at a time when con- 

 tinental land areas are glaciated. 



Unfortunately we as yet know practically nothing of the geologic 

 ages of the ancient elevated reefs of the Pacific and have as yet no 

 means for ascertaining their relation in time to those of the tropical 

 Atlantic. 



Guppy (1890, Trans. Victoria Institute, Vol. 23, p. 60) held the 

 opinion that the corals of the Great Barrier reef were growing on 

 the seaward edges of a submarine plateau, and Andrews (1902, 

 Proc. Linnean Soc. New South Wales, Part 2, pp. 145-185) shows 

 that the submerged continental shelf along the coast of Queensland 

 extends southward into the temperate regions, and that the more or 

 less detached coral patches which form the Great Barrier Reef 

 could not have formed the platform but merely grew here and there 

 along its seaward edge after the platform was submerged, and con- 

 ditions became favorable for coral growth. Indeed as I observed 

 in 191 3 the Barrier Reef ceases at the Murray Islands, while the 

 platform upon which it is growing extends northward to the shores 

 of New Guinea, but corals cannot grow in this region due to the 

 mud discharged from the Fly and other great rivers of Papua. 

 Thus this platform extends both northward and southward beyond 

 the limits of coral reef growth. Alexander Agassiz held the opin- 

 ion that the modern reefs of the Pacific were growing unconform- 

 ally upon the ancient limestone platforms and ledges. 



